Invalid Operation Exception Collection was modified C#

Everything works fine except when a user goes offline and i remove him from the collection it throws an exception .

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text.RegularExpressions;
using System.Threading;
using OpenQA.Selenium;
using OpenQA.Selenium.Edge;
using OpenQA.Selenium.Support.Events;
using OpenQA.Selenium.Support.UI;